The biosynthetic mechanism involved in porphyrin biosynthesis was studied. The mechanism of porphobilinogen transformation into uroporphyrinogens was examined with the help of synthetic tripyrranes. The obtained results suggest that uroporphyrinogen III is formed from prophobilinogen by a "three switch" mechanism in which the first unit of porphobilinogen condenses "head-to-head" with successive units of prophobilinogen followed by a migration of the "amino-methyl" unit. The synthesis of protoporphyrin XIII and of the tricarboxylic prophyrin formed by the action of coprogenase on coproporphyrinogen IV was achieved. Methods were developed for improving uroporphyrin and coproporphyrin synthesis. Prophobilinogen was oxidized in porphyric rats by prophobilinogen oxygenase, an inducible enzyme which diverts prophobilinogen from the porphyrin metabolic pathway. The oxygenase is an iron-sulfur containing protein of a strongly cationic nature. It associates to high molecular weight forms and has allosteric kinetics. By succinylation a succinylated enzyme is obtained which exists only in the low molecular weight from and reverts to classical kinetics. The enzyme was isolated from human erythrocytes. BIBLIOGRAPHIC REFERENCES: B. Frydman, R.B. Frydman, A. Valasinas, E.S. Levy and G. Feinstein. - BIOSYNTHESIS OF UROPORPHYRINOGENS FROM PORPHOBILINOGEN: MECHANISM AND THE NATURE OF THE PROCESS. Phil. Trans. R. Soc. Lond. B., 273, 137 (1976). A. Valasinas, E.S. Levy and B. Frydman. - SYNTHESIS OF 6'-AMINOETHYLTRIPYRRANES OF BIOSYNTHETIC INTEREST. Tetrahedron Lett., (5) 337 (1976).